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‘79 IT400F Stock Jetting

10 Sep 2020 21:28
YZBill
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‘79 IT400F Stock Jetting #1
Looking for the stock carb jetting configuration for a 1979 IT400F. Primarily the numbers I don’t have:
Main Jet - 350
Pilot Jet - 50
Needle Jet - ?
Jet Needle - ?
Slide - 3.0
Air Jet - ?

If anyone has a Factory or Clymer manual with this info, please reply here.
Thanks!

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11 Sep 2020 07:10
MarkT
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‘79 IT400F Stock Jetting #2
I have data that shows carb model as 2X800
Main Jet - 370
Pilot Jet - 60
Needle Jet - Q-0
Jet Needle - 6F16-2
Slide - ? (Part number shows 3.0)
Air Jet - ?

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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11 Sep 2020 20:33
YZBill
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‘79 IT400F Stock Jetting #3

MarkT wrote: I have data that shows carb model as 2X800
Main Jet - 370
Pilot Jet - 60
Needle Jet - Q-0
Jet Needle - 6F16-2
Slide - ? (Part number shows 3.0)
Air Jet - ?


Thanks Mark! I’m setting up my 79 YZ400F for racing AHRMA X-Country, so I’m de-tuning it to the 79 IT400F specs. The YZ and IT share the same cylinder. I’ve installed an IT piston, that has the windows vs. the cutout. The DG pipe I bought is for the IT400F/425G which fits the YZ frame, but DG has a different part number for the YZ400F pipe. Both ran the VM38 carb, but the jetting is different. What you posted is identical to the YZ400D/E jetting.

I’m curious to see how it will run with these changes and if I’ll need to make changes to the ignition timing. Years ago I rode an IT400 and it was an absolute tractor in the woods. You could lug it around all day in third gear, but a twist of the throttle was all you needed to tackle any obstacle.
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14 Sep 2020 08:22
YZBill
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‘79 IT400F Stock Jetting #4
MarkT, do you happen to have the ignition timing specs for the 79 IT400?

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14 Sep 2020 16:38
MarkT
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‘79 IT400F Stock Jetting #5
3.1mm BTDC is what my book says for the IT400F

(C, D, and E models 2.7mm)

I think in 1979 Yamaha had a more sophisticated... technology-wise... CDI box that allowed them to advance the static timing for more low-mid punch and then it would retard at high RPM to avoid meltdown.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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15 Sep 2020 16:08
YZBill
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‘79 IT400F Stock Jetting #6

MarkT wrote: 3.1mm BTDC is what my book says for the IT400F

(C, D, and E models 2.7mm)

I think in 1979 Yamaha had a more sophisticated... technology-wise... CDI box that allowed them to advance the static timing for more low-mid punch and then it would retard at high RPM to avoid meltdown.


Thanks again Mark. The 79 YZ400 and IT400 have the same static timing and use the same CDI box. The flywheels are different between, but the YZ400 uses the same flywheel as the 79 IT250 which also uses the same CDI box.

Now I’m looking at options for making the 79 YZ250 more “woods friendly”. I briefly rode it around my secret test track and it’s going to be a handful in the woods. Unfortunately it doesn’t share much with the IT250. Maybe some sort of flywheel weight for the button mag?

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